To quickly discover DNA variants of the causal genes involved in genetic disorders and to sequence all the bases in the exons and intron-exon boundaries of the six genes (PAH, PTS, GCH1, QDPR, PCBD1 and GFRP) involved in PKU and BH4 deficiency, 108 primer pairs were designed for multiplex PCR to amplify 39 exons containing 9,967 base pairs. Here, GCH1 is linked to phenylketonuria.